Cyclotron wave electron beam parametric amplifier



E. l. GORDON Oct. 29, 1963 CYCLOTRON WAVE ELECTRON BEAM PARAMETRICAMPLIFIER Filed April 19, 1962 2 Sheets-Sheet 1 FIG. I

PUMP

INVENTOR j E. l/CXDON Arrfi d M F1111 Oct. 29, 1963 E. l. GORDON3,109,146

CYCLOTRON WAVE ELECTRON BEAM PARAMETRIC AMPLIFIER Filed April 19, 1962 2Sheets-Sheet 2 //v I/ENTOR E. I. GORDON United States Patent Filed Apr.19, 1962, Ser. No. 188,718 7 Claims. ((31. ass-4.7

This invention relates to electron beam devices and more particularly tocyclotron wave parametric amplifiers.

One of the serious drawbacks of electron beam amplifiers, such as theklystron and traveling wave tube, has been the spurious noise whichnecessarily accompanies the generation of an electron beam. A recentimportant advance in the art is the cyclotron wave parametric amplifier,known also as the quadrupole amplifier, which, by employing theprinciples of fast cyclotron wave parametric amplification, permits thedirect removal of beam noise energy which would otherwise couple to thesignal wave during the amplification process.

The electron gun of this device produces a beam that flows successivelythrough an input coupler, a quadrupole amplifying resonator, and anoutput coupler. The beam is immersed in a uniform magnetic field whichestablishes a cyclotron frequency at which the electrons will rotate ifacted upon by forces transverse to the field.

The input coupler is a resonant circuit that is tuned to the cyclotronfrequency, and comprises a pair of parallel poles on opposite sides ofthe beam. When the input coupler is eXcited by a signal wave, electricfields are produced between the poles which are capable of exciting asignal cyclotron wave on the beam. Conversely, cyclotron wave noiseenergy at the signal frequency, which is inherent in the beam, istransferred to the input coupler and can thereafter be dissipated. Thepump resonator is excited by pump power which is usually, although notnecessarily, twice the cyclotron frequency; this pump power interactswith the beam to amplify the signal cyclotron wave. A necessarycondition for amplification is the production of quadrupole electricfields throughout the beam within the pump coupler; hence the termquadrupole amplifier. The output coupler is identical with the inputcoupler and it extracts the amplified low noise signal wave from thebeam.

One drawback of the cyclotron wave parametric amplifier is the fact thatthe pump resonator ordinarily can only be a half wavelength long. If thepump resonator is a full wavelength long, electric fields producedthroughout the beam will reverse direction at the midpoint of thecoupler. As a result, electrons that gain energy in the upstream half ofthe coupler will lose energy in the downstream half and vice versa, andthere will be no net amplification of the signal cyclotron wave. Thelength limitation of the pump resonator can be a very importantconsideration because the cyclotron wa ve gains energy as an exponentialfunction of distance. Additional gains can theoretically be achieved byusing two or more pump couplers between the input and output couplers.If this is done, however, each successive pump coupler must be verycarefully positioned so that successive pump fields are in spatialsynchronism. This type of amplification is also relatively inefficientbecause the exponential gain with respect to distance is successivelyterminated.

One type of full wavelength coupler is described in the United Statespatent application of A. Ashkin, Serial No. 126,938, filed July 26,1961. In this device, two poles on opposite sides of the beam aresubstituted for the quadrupole of the conventional pump resonator. Eachof the poles has a V-shaped surface along half of "ice its length and acurved surface over the other half. The change in configuration of thepoles compensates for the inherent reversal of the electric field. Thistype of resonator is relatively complicated and may present certainfabrication problems.

Accordingly, it is an object of this invention to increase the gainobtainable in a cyclotron wave parametric ampliher through the use of asingle pump resonator of simple structure.

More specifically, it is an object of this invention to provide a pumpresonator for a cyclotron wave parametric amplifier which may be a fullwavelength long or longer.

These and other objects of the invention are attained in an illustrativeembodiment thereof which comprises an electron gun for producing anelectron beam which flows successively through an input coupler, a pumpresonator that is excited from a source of pump frequency Wave energy,and an output coupler. According to one feature of the invention, thepump resonator comprises a hollow conductive cylinder which is a fullwavelength long at the pump frequency and which surrounds two rods thatare twisted through degrees in the form of a bifilar helix section. Theresonator is eX cited by pump energy in the mode, that is, theinstantaneous polarity of the cylinder is opposite that of the rods. Inaccordance with the invention, the twist of the rods compensates for theelectric field reversal which necessarily results from full wavelengthoperation.

The resonator also includes a pair of apertured end plates to which thehelical rods are attached. It is another feature of this invention thata slot he cut through the end plates of the resonator directly betweenthe two rods. As will be explained hereafter, this feature insuresexcitation in the mode.

These and other objects and features of the invention will be moreclearly understood from a consideration of the following detaileddescription taken in conjunction with the accompanying drawing in which:

.FIG. 1 is a perspective illustration of a cyclotron wave parametricamplifier employing the principles of my invention;

FIG. 2 is a sectional view of a microwave frequency pump resonator ofthe type used in the amplifier of FIG. 1;

FIG. 3 is a graph illustrating electric field intensity with respect todistance in the pump resonator of FIG. 1;

FIG. 4 is a view taken along line 44 of FIG. 2.

FIG. 5 is a view taken along line 5-5 of FIG. 2.

FIG. 6 is a view taken along line 6-6 of FIG. 2; and

FIG. 7 is a view taken along line 7-7 of FIG. 2.

Referring now to FIG. 1 there is shown an illustration of a cyclotronwave parametric amplifier comprising an electron gun 12, for forming andprojecting an electron beam toward a collector 13. A magnet (not shown)produces a longitudinal magnetic field B for focusing the beam andestablishing cyclotron modes of wave propagation within the beam. Thebeam is maintained within a substantial vacuum by an envelope 11 whichmay be of glass or other suitable material. An input coupler 15comprising a resonant circuit that is tuned to the cyclotron frequencymodulates the beam with signal frequency energy from source 16 andremoves fast cyclotron wave noise from the beam. The modulation resultsfrom transverse signal electric fields produced between parallel poles14 of coupler 15. Likewise, inherent beam noise excites transverseelectric field energy on poles 14 which is conveniently removed.

After modulation the signal frequency energy propagates as a cyclotronwave. The beam then passes through a pump resonator 17 which is normallytuned to twice the cyclotron frequency and is excited by pump power oftwice the cyclotron frequency from a source 18. The pump resonatortransfers energy from source 18 to the cyclotron mode of the beam,thereby amplifying the signal cyclotron wave. An output coupler 20comprising parallel poles 21 extracts the amplified signal frequencyenergy from the beam and transmits it to an appropriate load 22.

Although the input, pump, and output resonators are shown as portions ofa single block, each could constitute a separate and distinct element.FIG. 2 is a sectional view of a pump resonator of the type shown in FIG.1 and can be considered as being a separate and distinct element. Theresonator comprises an outer wall 23, two end plates 24 and two rods 25which are twisted through 180 degrees in the form of a bifilar helixsection. Central apertures 26 in the end plates 24 permit passage of theelectron beam. Pump resonator 17 is one wavelength long at the pumpfrequency. Normally the transverse electric field which would beproduced along the cavity axis would reverse direction at the midpointof the pump cavity as shown by curve 28 of FIG. 3.

As was pointed out in the aforementioned Ashkin application,conventional pump resonators cannot be a full wavelength long because ofthe electric field reversal illustrated in FIG. 3. When the beam entersa pump resonator roughly half of the electrons are in a proper phasecondition to gain energy while the other half loses energy. Normally, ifthe electric field reverses direction as shown in FIG. 3 the electronsthat gain energy along the upstream portion will lose energy in thedownstream portion because of the reversed direction of the electricfield. Hence, the conventional pump resonator is not capable of fullwavelength operation.

As will be appreciated from FIGS. 4 and 5, the 180 degree twist of rods25 fully compensates for the inherent electric field reversal in theresonator 17. FIG. 4 is a section taken along a plane which isapproximately onequarter the distance along resonator 17 while FIG. is asection taken along a plane which is approximately three-quarters thedistance along the resonator.

As will be described more fully hereafter, pump resonator 17 is excitedsuch that the instantaneous polarity of rods 25 is opposite that ofouter wall 23. It can easily be seen from FIG. 4 that this mode ofexcitation invariably produces quadrupolar electric fields E within theelectron beam. Assuming an instant in time in which rods 25 are at amaximum positive potential with respect to the outer wall, quadrupolarfields E will tend to focus the beam along an imaginary plane 29.

At plane 55 of FIG. 2, the relative polarities at the same instant oftime are just the opposite of those at plane 44 because of theaforementioned electric field reversal. As shown in FIG. 5, the rods 25are at a negative potential with respect to the outer wall. However,because of the intervening 90 degree twist of rods 25 between planes 4-4and 5-5, the quadrupolar electric fields E still tend to focus theelectrons along a plane 29 just as in FIG. 4. Hence, the electrons ofthe beam do not see the field reversal shown in FIG. 3. Those electronswhich are in a proper phase at plane 4-4 to gain energy are also in aproper phase to gain energy at plane 5-5. It can therefore beappreciated that the twist of rods 25 compensates for the field reversalwhich inherently results from full wavelength operation.

It should be noted that the angular rate of twist with distance of rods25 is just one-half the rate of change of the transverse electric field.As seen in FIG. 3, the electric field changes by 180 degrees betweenplanes 4-4 and 5--5, while rods 25 are twisted through 90 degrees alongthe same distance, as can be seen from FIGS. 4 and 5. Along the entirelength of pump resonator 17 the electric field goes through 360 degreeswhile rods 25 describe 180 degrees. From these considerations it can beappreciated that resonator 17 can be of any desired length. For example,if it were made two wavelengths long (720 .4 degrees), rods 25 would bemade to describe 360 degrees along its length.

Consider next the method of exciting resonator 17 in the proper mode ofoscillation. Referring to FIG. 6, it can be appreciated that regardlessof the instantaneous polarities of rods 25 and outer wall 23, themagnetic field lines H of the desired mode of oscillation are alwaystransverse to the axis of the resonator, as well as being transverse tothe instantaneous electric field lines. Magnetic coupling probe 31 istherefore positioned in a plane parallel to the axis of the resonator sothat when it is excited it can produce a magnetic field parallel withlines H and thereby support the desired oscillation mode.

The oscillation mode described, wherein the central rods 25 are of alike instantaneous polarity is known as the mode,as opposed to the modewherein the rods are of opposite instantaneous polarity. It is sometimespossible for magnetic coupling probe 31 to excite a mode in resonator 17because this mode is also characterized by a magnetic field that istransverse to the resonator axis. Excitation of the mode can beprevented by including slots 32 in plates 24 as shown in FIGS. 7 and 1.Referring to FIG. 7, it can be seen that the electric fields from theopposite poles 25 are in opposition along a plane 33 so that no verticalcurrents are transmitted across plane 33 on either of the end plates 24.Insertion of slot 32 in end plate 24 does not therefore affect the mode.The mode, however, is supported by end plate current flowing between theoppositely polarized rods 25. Slots 32 constitute an open circuit withrespect to such currents and thereby prevent the formation of a mode.

It should be pointed out that slots 32 are not always necessary for thesuccessful formation of a mode. If the magnetic coupling loop 3 1 islocated directly between poles 25 as shown in FIG. 6, there may be noexcitation of the mode because this mode does not have a large magneticfield component that is transverse to the plane of the coupling loop atthat position. The slots are nevertheless desirable to insureoscillation stability in the resonator.

The above-described devices are intended only to be illustrative of myinvention. Numerous other arrangements may be made by those skilled inthe art without departing from the spirit and scope of the invention.

What is claimed is:

1. A cyclotron wave parametric amplifier comprising:

means for forming and projecting an electron beam along a path;

means for producing a magnetic field along said path therebyestablishing a cyclotron mode of Wave propagation within the beam;

means for causing signal frequency energy to propagate within said beamas a cyclotron wave;

said last-mentioned means comprising a first cavity resonatorsurrounding a first portion of said path and having a pair of parallelpoles on opposite sides of the path;

a source of pump frequency energy;

said pump frequency being substantially twice said signal frequency;

a pump resonator surrounding a second portion of said path;

said pump resonator comprising a conductive cylinder which surrounds twoconductive rods;

said rods being twisted through substantially degrees in the form of abifilar helix;

means for exciting the pump resonator substantially only in the modewith pump frequency energy;

and means comprising a third cavity resonator for extracting signalfrequency energy from said beam.

2. An electron beam device comprising:

means for forming an projecting an electron beam along a path;

means adjacent said path for modulating said beam in a cyclotron mode;

means adjacent said path for demodulating said beam;

a cavity resonator located adjacent said path and be tween saidmodulatint means and said dcmodulating means;

said resonator comprising a conductive cylinder which surrounds twoconductive rods;

an end plate on each end of said cylinder;

an aperture in each of said end plates for permitting passage of saidbeam;

said conductive rods being attached to said end plates on opposite sidesof said aperture and describing helix segments around a portion of saidpath;

the number of turns of the helix segments being substantially equal tohalt the number of wavelengths along the length of the cavity resonatorat its resonant frequency;

and means for exciting the cavity resonator substantially only in thei--}- mode with electromagnetic wave energy.

3. Electron beam pumping apparatus comprising:

a source of pump frequency energy;

a resonator which is substantially one wavelength long at said pumpfrequency;

said resonator comprising an outer wall Which surrounds two conductiverods;

said rods being twisted through 186 degrees in the form of a bifilarhelix;

and means for exciting said resonator substauti-aly only in the modewith energy from said source.

4. An electron beam device comprising:

\means for forming and projecting an electron beam 'along a path;

means for producing a magnetic field along said path therebyestablishing a cyclotron anode of wave propagation within the beam;

means for causing signal frequency wave energy to propagate along saidbeam as a cyclotron wave;

a source of pump frequency energy;

said pump frequency being approximately twice the signal frequency;

a pump resonator comprising a conductive cylinder surrounding a portionof said path and having a pair of end plates with apertures in thecenters thereof for permitting passage of said electron beam;

two conductive rods attached to said end plates on opposite sides ofsaid apertures;

said rods describing helix segments around said path portion;

means comprising a magnetic coupling probe located in a planesubstantially parallel with the central axis of said cylinder forexciting said pump resonator substantially only in the mode witn energyfrom said pump frequency source;

said pump resonator being :1 pump frequency wavelengths long;

said helix segments each describing n times 180 degrees of helicalrotation along said path portion;

and means for extracting signal frequency wave energy from said beam.

5. Electron beam pumping apparatus comprising:

a source of pump frequency energy;

a resonator which is n Wavelengths long at said pump frequency and whichhas a central axis;

said resonator comprising an outer Wall and two end plates;

said end plates having apertures along said central axis;

two helical rods surrounding said central axis and being attached attheir ends to said end plates on opposite sides of said apertures;

said helical rods describing substantially 'n/ 2 revolutions betweensaid end plates;

and means for exciting said resonator substantially only in the modewith energy from said source.

6. An electron beam parametric amplifier comprising:

means for forming and projecting an electron beam along an extendedpath;

means for producing a magnetic field along said path therebyestablislihr cyclotron modes of wave propagation within said beam;

means adjacent said path for modulating said beam in a cyclotron mode;

means adjacent said path for demodulating said beam;

a source of pump frequency energy;

a cavity resonator located between said modulating and demodulatingmeans;

said resonator comprising a conductive cylinder which is coaxial withsaid path;

a pair of end plates on opposite ends of said cylinder each of whichhave an aperture along said central two helical rods surrounding saidcentral axis and being attached at their ends to said end plates onopposite sides of said apertures;

said helical rods each describing x revolutions between said end plates;

said resonator being substantially 2x wavelengths long at said pumpfrequency; and

means for exciting said resonator substantially only in the mode withpump firequency energy.

7. The parametric amplifier of claim 6 wherein each of said end platescontains a slot which is substantially perpendicular to a lineconnecting the points at which said rods are connected to the respectiveend plates.

No references cited.

3. ELECTRON BEAM PUMPING APPARATUS COMPRISING: A SOURCE OF PUMPFREQUENCY ENERGY; A RESONATOR WHICH IS SUBSTANTIALLY ONE WAVELENGTH LONGAT SAID PUMP FREQUENCY; SAID RESONATOR COMPRISING AN OUTER WALL WHICHSURROUNDS TWO CONDUCTIVE RODS;